A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In a lithographic apparatus, the displacement of objects such as a substrate or a patterning device such as a reticle, are required. Both comparative large displacements in one or two directions and comparative small displacements for accurate positioning are required. Both requirements are often realized by combining a so-called long stroke motor capable of displacing an object over comparatively large distances in one or two directions with a so-called short stroke motor comprising one or more linear actuators, capable of displacing an object with high accuracy over comparatively small distances. Typical values may be that the long stroke motor has a stroke of ˜500 mm with micrometer precision while the linear actuator(s) allows a stroke of a few mm with nanometer accuracy. By mounting the short stroke motor on the long stroke motor, an object that is held by an object table connected to said short stroke motor can both be displaced over large distances and accurately positioned. A particular example of such a long stroke motor is a planar motor as described in United States patent U.S. Pat. No. 6,496,093 or United States patent U.S. Pat. No. 6,441,514, incorporated herein in their entirety by reference. A planar motor typically comprises a magnet plate and a coil assembly, one of the plate and assembly being movable relative to the other of the plate and assembly. For example, the planar motor may have a stationary magnet plate and a movable coil block comprising a plurality of coil sets. By applying the appropriate currents to the different coil sets, forces can be generated between the coil block and the magnet plate. Those forces can displace the object table connected to the coil block in a first direction parallel to the plane of the magnet plate, a second direction parallel to the plane of the magnet plate and perpendicular to the first direction and in a third direction perpendicular to said plane. In general, the forces parallel to the plane of the magnet plate are applied to displace the object table distances and/or angles in the horizontal plane (X, Y and Rz) while the forces in a direction perpendicular to said plane are generated to maintain the object table at a predetermined height and inclination (Z, Rx and Ry).
In order to allow displacements in said first direction and said second direction orthogonal to said first direction over comparatively large distances, the magnet plate is designed in such a way that it comprises a periodically alternating magnetic field in those two orthogonal directions. A coil that extends in said first direction and is displaced in said second direction will therefore encounter a periodically alternating flux linkage. When the same coil is rotated 90 degrees i.e. that it extends in said second direction and is displaced in said first direction coil will also encounter a periodically alternating flux linkage. When the coils are provided with a current, a force is generated between the coil and the magnet plate, the force being proportional to the current and to the variation of the flux linkage.
By applying a multi-phase winding and providing the appropriate currents to the different phases, a constant vertical force (in order to compensate for the weight of the object table) and a horizontal force (in order to displace the object table to the required position and/or orientation) can be generated between the magnet plate and the coil block provided with the multi-phase winding. By applying two multi-phase windings displaced orthogonal to each other in said first and second direction, displacements along said first and second direction are made possible. In general, the coil block is equipped with more than two multi-phase windings or coil units. By an appropriate arrangement of the different coil units, the planar motor allows positioning of the object table in up to and including six degrees of freedom. Realizing the required displacements and positioning in up to and including six degrees of freedom is done by providing the appropriate currents to the different coils in the different coil units. As a consequence, dissipation will occur in the different windings.